Django:在多对多关系之前，实例需要有一个主键值

This is my model

这是我的模型

class Business(models.Model):
    business_type = models.ManyToManyField(BusinessType)
    establishment_type = models.ForeignKey(EstablishmentType)
    website = models.URLField()
    name = models.CharField(max_length=64)

    def __unicode__(self):
        return self.name
class Business(mode